This disclosure relates to Toll-like receptor 7 (“TLR7”) agonists and conjugates thereof, and methods for the preparation and use of such agonists and their conjugates.
Toll-like receptors (“TLRs”) are cell-surface receptors that recognize pathogen-associated molecular patterns (“PAMPs”). Activation of a TLR by the binding of a corresponding PAMP signals potential infection by a pathogen and stimulates the immune system to fight the infection. Humans have 11 TLRs, named TLR1 through TLR11.
The activation of a TLR—with TLR7 being the most studied—by an agonist can have an adjuvant effect on the action of vaccines and immunotherapy agents in treating a variety of conditions other than actual pathogen infection, by stimulating the immune response.
TLR7 recognizes PAMPs associated with single-stranded RNA viruses. Its activation induces secretion of Type I interferons such as IFNα and IFNβ (Lund et al. 2004). It has two binding sites, one for single stranded RNA ligands such as ssRNA40 (Berghöfer el al. 2007) and one for guanosine (Zhang et al. 2016).
TLR7 can bind to, and be activated by, guanosine-like synthetic agonists such as imiquimod, resiquimod, and gardiquimod, which are based on a 1H-imidazo[4.5-c]quinoline scaffold.

Synthetic TLR7 agonists based on a pteridinone molecular scaffold are also known, as exemplified by vesatolimod (Desai el al. 2015), which has been in Phase 2 clinical trials. The potency of vesatolimod is reported to be 100× less than that of the corresponding purine-8-one compound, as measured by IFN-α induction (Roethle et al. 2013).

Other synthetic TLR7 agonists are based on a purine-like scaffold, frequently according to formula (A):
where R, R′, and R″ are structural variables, with R″ typically containing an unsubstituted or substituted aromatic or heteroaromatic ring.
Disclosures of bioactive molecules having a purine-like and their uses in treating conditions such as fibrosis, inflammatory disorders, cancer, or pathogenic infections include: Akinbobuyi et al. 2015b and 2016; Barberis et al. 2012; Carson et al. 2014; Ding et al. 2016, 2017a, and 2017b; Graupe et al. 2015; Hashimoto et al. 2009; Holldack et al. 2012; Isobe et al. 2009a and 2012; Jin et al. 2017a and 2017b; Peterson 2014; Pryde 2010; and Seifert 2015.
The group R″ can be pyridyl: Bonfanti et al. 2015a and 2015b; Halcomb et al. 2015; Hirota et al. 2000; Isobe et al. 2000, 2002, 2004, 2006, 2009a, 2011, and 2012; Kasibhatla et al. 2007; Koga-Yamakawa et al. 2013; Musmuca et al. 2009; Nakamura 2012; Ogita et al. 2007; and Yu et al. 2013.
Bonfanti et al. 2015b discloses TLR7 modulators in which the two rings of a purine moiety are spanned by a macrocycle:

A TLR7 agonist can be conjugated to a partner molecule, which can be, for example, a phospholipid, a poly(ethylene glycol) (“PEG”), or another TLR (commonly TLR2). Exemplary disclosures include: Carson et al. 2013, 2015, and 2016, Chan et al. 2009 and 2011, Lioux et al. 2016, Maj et al. 2015, Ban et al. 2017; Vemejoul et al. 2014, and Zurawski et al. 2012. Conjugation to an antibody has also been disclosed: Akinbobuyi et al. 2013 and 2015a, and Gadd et al. 2015. A frequent conjugation site is at the R″ group of formula (A).
TLR7 agonists based on a 5H-pyrrolo[3,2-d]pyrimidine scaffold have also been disclosed. See Cortez et al. 2017a and 2017b, McGowan et al. 2017, and Li et al. 2018.
Jensen et al. 2015 discloses the use of cationic lipid vehicles for the delivery of TLR7 agonists.
Some TLR7 agonists, including resiquimod are dual TLR7/TLR8 agonists. See, for example, Beesu et al. 2017; Lioux et al. 2016; and Vemejoul et al. 2014.
TLR7 agonists based on a 5H-pyrrolo[3,2-d]pyrimidine scaffold have also been disclosed. See Cortez et al. 2017a and 2017b, McGowan et al. 2017, and Li et al. 2018.
Full citations for the documents cited herein by first author or inventor and year are listed at the end of this specification.